A method called an MS/MS analysis (tandem MS analysis) is widely used as one of the mass spectrometric techniques for identification, structural analyses or quantitative determination of compounds having large molecular weights. There are various configurations of mass spectrometers designed for performing MS/MS analyses, among which the triple quadrupole mass spectrometer is comparatively simple structured as well as easy to operate and handle.
In a generally used triple quadrupole mass spectrometer, sample-derived ions generated in an ion source are introduced into a front quadrupole mass filter (which is often commonly represented as “Q1”). In the front quadrupole mass filter, an ion having a specific mass-to-charge ratio is selected as a precursor ion. This precursor ion is introduced into a collision cell containing an ion guide with four or more poles (this ion guide is often commonly represented as “q2”). A collision-induced dissociation gas, such as argon, is supplied to this collision cell, and the precursor ion collides with this CID gas in the collision cell, whereby the ion is fragmented into various kinds of product ions. These product ions are introduced into a rear quadrupole mass filter (which is often commonly represented as “Q3”). In the rear quadrupole mass filter, a product ion having a specific mass-to-charge ratio is selected. The selected product ion reaches a detector and is thereby detected.
Such a triple quadrupole mass spectrometer can be used independently. However, this device is often used in combination with a chromatograph, such as a gas chromatograph (GC) or liquid chromatograph (LC). In particular, in recent years, the gas chromatograph triple quadrupole mass spectrometer (which is hereinafter called the “GC-MS/MS” according to common practice) and liquid chromatograph triple quadrupole mass spectrometer (which is hereinafter called the “LC-MS/MS” according to common practice) have become vital devices in the field of microanalysis which is aimed at analyzing a sample containing a large number of compounds or a sample mixed with various foreign substances, as in the case of testing residual pesticides in foodstuffs, testing environmental pollutants, checking the concentration of medicinal chemicals in blood, or screening drugs or poisonous substances.
In general, chromatograph triple quadrupole mass spectrometers have multiple measurement modes for the MS/MS analysis, such as the MRM (multiple reaction monitoring) measurement mode, precursor ion scan measurement mode, product ion scan measurement mode, and neutral loss scan measurement mode (for example, see Patent Literature 1).
In the MRM measurement mode, the mass-to-charge ratio of the ion which is allowed to pass through the mass filter is fixed in each of the front and rear quadrupole mass filters so as to measure the intensity (amount) of a specific product ion generated by the fragmentation of a specific precursor ion.
In the precursor ion scan measurement mode, while the mass-to-charge ratio of the ion (product ion) which is allowed to pass through the rear quadrupole mass filter is fixed, the mass-to-charge ratio of the ion (precursor ion) which is allowed to pass through the front quadrupole mass filter is continuously varied over a predetermined range of mass-to-charge ratios. By this mode, a mass spectrum of various precursor ions which generate a specific product ion by the fragmentation within the collision cell can be obtained.
In the product ion scan measurement mode, while the mass-to-charge ratio of the ion (precursor ion) which is allowed to pass through the front quadrupole mass filter is fixed, the mass-to-charge ratio of the ion (product ion) which is allowed to pass through the rear quadrupole mass filter is continuously varied over a predetermined range of mass-to-charge ratios. By this mode, a mass spectrum of various product ions generated by the fragmentation of a specific precursor ion within the collision cell can be obtained.
In the neutral loss scan measurement mode, both the mass-to-charge ratio of the ion which is allowed to pass through the front quadrupole mass filter and that of the ion which is allowed to pass through the rear quadrupole mass filter are continuously varied in an interlocked fashion so as to constantly maintain the difference in the mass-to-charge ratio between the two ions respectively allowed to pass through the front and rear quadrupole mass filters (i.e. the neutral loss). By this mode, it is possible to investigate the combination of the precursor ion and product ion which produces a specific neutral loss by the fragmentation within the collision cell.
Each of the previously described measurement modes is appropriately used according to the purpose of the analysis, kind of sample, kind of compound to be analyzed, and other factors. For example, the precursor ion scan measurement mode is useful in the case of specifically investigating various compounds having a specific partial structure, since this mode enables the exhaustive detection of precursor ions having a specific partial structure resulting from the fragmentation of the original compound. In particular, in the case of a triple quadrupole mass spectrometer equipped with an ion source employing the electron ionization method in which a fragmentation associated with the ionization (i.e. the so-called “in-source decay”) is likely to occur, the product ion scan measurement mode can be conducted as follows: An ion having a specific partial structure resulting from the fragmentation in the ion source is selected as the precursor ion. This precursor ion is further fragmented within the collision cell, and a mass spectrum of the generated product ions is obtained. By investigating the pattern of this mass spectrum, it is possible to distinguish between structural isomers, positional isomers or similar compounds which are identical in molecular weight and only different in chemical structure.
As noted earlier, the precursor ion scan measurement and product ion scan measurement are each useful for the estimation of the chemical structure of a compound. Therefore, in the case of identifying an unknown compound with a complex chemical structure or estimating its structure by means of a GC-MS/MS or LC-MS/MS, a method is widely used in which a precursor ion scan measurement or product ion scan measurement of a plurality of characteristic ion species originating from the compound concerned is performed and the spectrum patterns of a plurality of mass spectra obtained as a result of the measurements are compared with the spectrum patterns of the mass spectra collected in a database to determine the degree of matching and thereby identify the compound.
FIGS. 6A-6C show the procedure of such a conventional compound identification process. In the present example, three kinds of ions (m/z 155, m/z 158 and m/z 284) characteristic of the compound species which the compound to be identified belongs to are specified as the precursor ions, and the product ion scan measurement is performed. The signal intensities of the product ions obtained in each product ion scan measurement are added to create a chromatogram (total ion chromatogram), on which a peak appears at a retention time of the compound to be identified, as shown in FIG. 6A. In other words, if the retention times of the peaks detected on the respective chromatograms agree with each other, it is possible to estimate that those peaks are formed by the ions derived from the same compound. Accordingly, at the location where these peaks appear, or typically, at the peak-top position of these peaks, a mass spectrum (product ion spectrum) is extracted from each peak as shown in FIG. 6B, and each mass spectrum is individually compared with the database in which mass spectra of known compounds are collected. Consequently, as shown in FIG. 6C, three results of the database search are obtained. The analysis operator checks these search results and eventually identifies the compound, for example, by locating the compound candidates which have been found in all of these results.